The present invention concerns a system and a method for wireless communication between a plurality of transceivers, arranged respectively in a plurality of delimited spaces each having at least one entrance, and portable electronic units provided with means of radio-communication with said plurality of transceivers, particularly with a view to detecting each of said electronic units which penetrate one of said delimited spaces. xe2x80x9cEntrancexe2x80x9d is used broadly to mean any door or pathway giving access to a delimited space concerned.
The present invention concerns more particularly a system and a method for communication between persons or objects provided with said portable electronic units and a plurality of transceivers arranged in a plurality of neighbouring delimited spaces or spaces located in proximity to each other. It will be noted that the delimited spaces may be fixed locations, such as delimited zones of a fun fair or a sports centre, or mobile locations such as compartments or carriages of a means of public transport.
A system for detecting individuals or objects provided with transponders or electronic tickets is known from European Patent Application No. 1,065,635. According to the method disclosed in this document, a low frequency communication is provided at the entrance to the delimited space, particularly of the order of a hundred KHz, allowing localised communication between a transmitter or reader and the portable transponders. Then, a high frequency communication is provided between a high frequency transmitter, particularly of the order of hundreds of MHz or more, located inside the delimited space and the transponders located in the space when the doors providing access to the space are closed. However, a high frequency communication may occur at any time as soon as the transponders are awakened and the distance between the transponders and a fixed transceiver of the delimited space is less than the maximum communication distance. Thus, such high frequency communication may also be generated upon exiting the delimited space.
The transponders used are generally of the active type, i.e. including their own energy source. These active transponders or electronic tags or contact-less electronic cards are hereinafter called xe2x80x9cportable electronic unitsxe2x80x9d. As is shown in European Patent Application No. 1,065,625, the portable electronic units are provided with low frequency communication means and high frequency communication means. Preferably, the electronic units are in standby at a low frequency and are awakened upon receiving a low frequency signal transmitted in the entrance-exit regions, the high frequency communication following in accordance with a given communication protocol.
In the case of the present invention, the inventors have detected a problem linked to high frequency communication when access to and/or the presence in several neighbouring delimited spaces has to be monitored, as is shown in FIG. 1. This Figure shows schematically and partially the carriages W1 and W2 of a first train 2 and the carriages W3 and W4 of a second train 4 located on a neighbouring parallel track, as can occur particularly in a train station. High frequency transceivers 6 to 11 are arranged inside the carriages. In an entirely schematic way, the zones inside which portable electronic units 20, 22 can receive a signal from one of transceivers 6 to 11 are represented by circles 12 to 17. Likewise, the regions in which units 20 and 22 transmit a high frequency signal able to be received by the transceivers are schematically represented by circles 24 and 26 in dashed lines.
It is clear from FIG. 1 that portable unit 20 can have a two-directional high frequency communication with transceivers 10 and 11 located in two adjacent carriages of train 4. Portable unit 22 located in carriage W3 can have a two-directional communication with transceivers 7 and 10 located respectively in carriages W1 and W3 belonging respectively to the two trains 2 and 4. It will thus be understood that, in the absence of other precautions, communications may be established between a portable unit located in a carriage and fixed transceivers located in other carriages. This may lead, on the one hand, to undesired communications and in particular to an erroneous detection of the presence of persons or objects in a given delimited space. On the other hand, even if precautions are taken to avoid any erroneous detection, in particular by the presence of low frequency transmitters or readers arranged at the entrances to the delimited spaces, the different electromagnetic fields transmitted by the various fixed transceivers and by the portable electronic units generate undesired interactions and in particular jamming of the communications provided when all of the transceivers are operating at the same frequency. This solution appears necessary however on first analysis given that the portable units can enter one or the other of the delimited spaces, without this being predetermined, since the communication system is used in particular, in a preferred embodiment, for detecting and/or localising people in means of transport or delimited neighbouring zones of a building.
One object of the present invention is to resolve the problem explained hereinbefore by providing an efficient communication system and method able in particular to be used for detecting people provided with portable electronic units and able to enter any one of a plurality of delimited spaces provided with high frequency communication means, for example greater than 3 MHz.
The invention therefore concerns first of all a communication system as defined previously characterised in that the plurality of transceivers and the portable electronic units are arranged so as to be able to communicate, among a set of given frequencies and or sub-sets of frequencies respectively associated with said frequencies, at any one frequency and/or frequencies of an associated sub-set of frequencies, and in that each of said transceivers is associated with means for occupying a frequency exclusive thereto, said transceivers including:
listening means arranged to determine whether an occupied signal is present on any frequency among said set of frequencies and/or whether a given amplitude threshold is exceeded upon reception of said frequency;
transmission means arranged to transmit at any frequency of said set of frequencies; and
means for selecting, in a period prior to a period of communication with portable electronic units, an exclusive frequency among said set of frequencies which does not contain an occupied signal or whose received amplitude level is less than said threshold.
Transmission means also transmit at the selected exclusive frequency an occupied signal for said exclusive frequency at least periodically during at least said communication period, this communication system having means for communicating information relative to said exclusive frequency to the portable units entering and/or exiting and/or located in the delimited space inside which the transceiver being considered is arranged prior to a high frequency communication being established. In the present description, xe2x80x9coccupied signalxe2x80x9d means a signal indicative of the occupied state of a frequency, and xe2x80x9cexclusive frequencyxe2x80x9d means a frequency which is exclusive to a transceiver.
In a preferred embodiment, the listening means and the transmitting means are alternately activated at any frequency of said set of frequencies to determine whether this frequency is available and to signal intention to occupy it. In a preferred variant, the selection of an exclusive frequency for a transceiver only occurs after several listening means and transmission means activation alternations, without such frequency being received by the listening means. To prevent the exclusive frequency occupying means associated with two neighbouring delimited spaces which are switched on simultaneously, particularly when the doors of a means of transport are opened, causing the transmission and listening periods to be superposed, the activation period of the listening means is preferably different for occupying means associated with transceivers arranged in neighbouring delimited spaces.
Thus, in order for the system according to the invention to operate, the portable electronic units have to be arranged so as to be able to communicate at any frequency among said aforementioned set of frequencies or frequencies of frequency sub-sets associated with said frequencies, as will be explained in more detail hereinafter. Those skilled in the art know how to design such units wherein the communication frequency can be adjusted and selected from among several predetermined channels.
In a preferred embodiment, the exclusive frequency selected by one or more transceivers located in a given delimited space is communicated to the portable units entering and possibly exiting by one or more low frequency transmitters arranged at the entrances to the plurality of delimited spaces associated with the communication system according to the invention. The low frequency communication is localised in the region of the entrances so that only the portable units which pass through the entrances, in one direction or the other, can receive information as to the exclusive frequency selected. Thus, when an electronic unit is awakened by a low frequency transmitter located in proximity to an entrance to a given delimited space, the transmitter sends an information signal concerning which of the high frequency communication channels has to be used subsequently in accordance with a predefined communication protocol.
The invention also concerns a method for communication between a plurality of transceivers, arranged respectively in delimited spaces next to each other or capable of being so at least temporarily and portable electronic units capable of penetrating such spaces. This method provides the following steps for each transceiver in a period prior to a high frequency communication period:
listening at a first frequency of said set of frequencies to determine whether an occupied signal is present on this first frequency and/or whether a given amplitude threshold is received for this first frequency;
changing channel to listen at another frequency of said set to determine whether an occupied signal is received on said first frequency or whether the amplitude level received for this first frequency is greater than the threshold; until a frequency is available, which is then selected as the exclusive frequency. This method also includes the following steps:
sending an occupied signal for said selected exclusive frequency, so that it is not subsequently taken by transceivers arranged in neighbouring delimited spaces, at least during a subsequent period of communication at the selected exclusive frequency and/or a frequency of a sub-set of frequencies associated with this exclusive frequency;
communicating the exclusive frequency selected for a high frequency communication to the electronic units entering and/or exiting and/or located in the delimited space being considered.
According to a preferred variant, as soon as an exclusive frequency is selected, the transmission means continue to transmit an occupied signal at this exclusive frequency without interruption, periodically or continuously, until the beginning of said communication period and throughout said period.
As a result of the communication method according to the invention, it is possible to select in a reliable manner, an available frequency as the exclusive communication frequency for a transceiver or transceivers associated with a same delimited space. Indeed, the method according to the invention efficiently determines whether a frequency is available and correctly informs other occupying means associated with other transceivers of the fact that a given frequency has been selected to avoid these other occupying means selecting a frequency which has already been taken.
Other features and advantages of the present invention will also appear from the following description, made with reference to the annexed drawings, given by way of non-limiting example, in which: